This invention relates to a vacuum transport belt for providing improved registration of paper in a multipass, color raster line printer and the like. The vacuum transport belt according to the present invention registers the paper firmly and accurately with a previously unattainable degree of invariance with respect to a writing head and a series of toning heads, while at the same time moving the paper relative to the stationary writing and toning heads.
The typical prior art color electrostatic plotters, which are of multipass nature, rely on a novel but unreliable system to achieve adequate color to color registration. This system consists of writing tick marks during the first black pass along both edges of the paper. During subsequent passes an optical system reads these marks and feeds back any dimensional or position of changes in the paper to the writing control system and paper tracking system. This method works well for correcting the typical positional movement of the paper, but it cannot correct for paper growth in the direction parallel to the writing head. The registration marks of prior art systems that are plotted along the edges of the paper are unsightly compromises. Also, prior art web based plotters require significant operator time to precisely align and thread the paper into the plotter every time a new plot is to be generated. The necessity of printing registration marks also has the disadvantage of generating wasted paper every time a new plot is completed.
The advent of the single pass web electrostatic plotter was an attempt to solve the registration problem and increase plotting speed. However, this method has the disadvantage of placing very tight constraints on the toning system. In order to take advantage of the high potential output speed, very little time is allowed for the toners to dry adequately. The single pass system also typically requires four expensive writing heads and four vector to raster controllers instead of the single head and single controller required by the multipass system. Registration can still be a problem, unless the four writing heads are very precisely lined up relative to each other and the paper does not move significantly during the printing process.
A more recent attempt at solving the paper stabilizing problem was partially successful with the advent of a color electrostatic plotter employing a rotating drum and helically scanning head and toning system. The salient advantage of the helical scanning drum system is that cut sheet paper is loaded on the drum automatically and held down by vacuum, while a small writing head and toning shoe helically write the image on paper. This system has a disadvantage of a relatively small writing head and toning system which must be very precisely and compliantly mounted to the traveling carriage. Further disadvantages to helical scan plotting are overwhelming. For example, the adjacent band to band writing and toning "edge effects" are very difficult, if not impossible, to overcome. The rigid drum acting as a backing surface creates a very harsh environment for the dielectric paper. Excessively high head pressures are required to achieve adequate writing quality, thereby resulting in damage to the dielectric coating of the paper.
In view of the foregoing disadvantages of prior art devices, it is apparent that what is needed is a paper transport system for an electrostatic plotter or multipass color raster line printer which enables precise and invariant registration of paper against a writing head and series of toning heads while the paper is moved back and forth over the writing and toning assembly. Precise registration of the paper is necessary in order to obtain proper resolution and an improved degree of color to color registration. It is known that superior and consistent color rendition from a raster type plotter requires a pass to pass registration of better than one half the dot spacing. At typically 400 dots per inch, the accuracy required is 30 micrometers. Since dielectric paper (or any unsealed paper) grows and shrinks with very small changes of humidity during a plotting cycle, it is obvious that it is very important to keep the paper stabilized during subsequent plotting passes. Another very important requirement of the electrostatic plotting process is that the head to paper interface must be very intimate (.+-.1 micrometer) across the whole width of the paper, without applying excessive pressure.